In a process for manufacturing an electrode coated film for lithium ion battery, an electrode coated film for photovoltaic power generation, and the like, there has been a drying method suitable for heat-drying workpieces with a coated film formed on a sheet surface thereof. The drying method is performed such that a plurality of infrared heaters 50 arranged in a furnace body 10 undergoes hot-air heating or infrared heating while the workpieces are moved inside of the furnace as shown in FIG. 1. As the infrared heater 50 used in such a furnace, a heater having a protection tube 52 around a filament 51 at a center as shown in FIG. 2 has been widely used (Patent Document 1). The reference number 53 refers to a supporting body of the filament 51.
Water, organic solvent and the like having intermolecular hydrogen bonds are included in components of a coated film. Due to this, in order to increase productivity of the coated film drying furnace, it is necessary to radiate a large quantity of heat in the furnace from the infrared heaters 50, and cause the water and organic solvent contained in the work to quickly evaporate.
Thus, conventionally, in a case of using the heater shown in FIG. 2, a method of rising a temperature of the filament 51 of the infrared heater 50 and increasing radiant energy thereof has been generally used. When the temperature of the filament 51 is risen, as shown in FIG. 3, it is known that a peak of a radiation spectrum transfers to a short wavelength side. Especially when the temperature of the filament 51 is made to be at 700° C. or more, a dominant wavelength of the radiation spectrum comes to 3.5 μm or less that is a near infrared range. Such a near infrared ray is said to have superior ability to cut off the intermolecular hydrogen bonds that hinder the evaporation; therefore, rising the temperature of the filament 51 of the infrared heater 50 is effective also from this respect. Note that, in the present description, a range in which the wavelength is at 3.5 μm or less will be referred to as the near infrared range.
However, when the filament temperature of the infrared heater 50 is risen, a temperature of the protection tube 52 surrounding a circumference thereof gradually rises, whereby the protection tube 52 itself becomes a radiator and radiates infrared rays. For example, when the temperature of the protection tube 52 is 300° C., as shown in FIG. 3, the work and furnace walls can be heated by the infrared rays with the dominant wavelength of 5 μm being radiated within the furnace. However, under such a condition, an amount of the radiant energy of the near infrared range of 3.5 μm or less as aimed is very small. Due to this, the hydrogen bonds cannot be cut. When the radiant energy of 3.5 μm or less is to be increased, radiant energy in a far infrared range also increases, whereby the work and the furnace walls are overheated. Further, the temperature may go beyond an ignition temperature of the evaporated organic solvent, and an explosion may occur.
Note that Patent Documents 2 to 4 describe infrared heaters for heating fluids. The heater of Patent Document 2 is a halogen heater and is inserted at a center of a transparent quartz tube. This transparent quartz tube has an inlet opening and outlet opening for gas to be heated and heats the gas flowing therein. Further, Patent Document 3 describes an infrared ray element that inserts a radiation tube, in which a tungsten heater is sealed inside a silica glass tube, into a cooling tube formed of silica glass and that performs heating while flowing liquid or gas to be heated in a passage formed between the radiation tube and the cooling tube. Further, Patent Document 4 describes a liquid heater that inserts a second hollow tube, in which a halogen lamp is sealed, into a first hollow tube having a flow-in section and a flow-out section for a fluid, and heats the fluid inside the first hollow tube. However, these are all heaters for heating the fluid flowing in a flow passage surrounding the heater and are not for heating a work inside a furnace.
Other than this, Patent Document 5 describes a furnace that provides a quartz protection tube at a center of a furnace body, puts an object to be heated therein, and performs heating to a high temperature of about 2000° C. by four infrared heaters arranged around the quartz protection tube. Cooling air is used to prevent protection tubes covering outer surfaces of the heaters from softening and deforming. However, Patent Document 5 also is for heating the contents of the quartz protection tube and is not for heating a work in a furnace, and in addition, a temperature range is completely different.
Further, Patent Document 6 discloses a vapor phase epitaxy device in which a double-tube type heater is arranged in a reaction chamber. This double-tube type heater reduces a surface temperature by cooling a space between an outer tube and an inner tube by air and prevents unnecessary deposition of reactant on a heater surface, and in addition mitigates thermal stress on a quartz glass configuring the outer tube. However, what is disclosed therein is a batch-type vapor phase epitaxy device, and is not a furnace for continuously drying a coated film having an absorption spectrum for electromagnetic waves of 3.5 μm or less and including water and organic solvent having the hydrogen bonds and the like. Further, in this vapor phase epitaxy device, walls inside the furnace are indirectly cooled by water, and energy radiation thereof is large; therefore, it is not economical for a large-scale continuous furnace. Accordingly, the prior art documents searched by the applicant do not disclose an art for efficiently drying a coated film having hydrogen bonds while suppressing a rise in temperature inside a furnace.